2.1 What are the units for measuring sound

One parameter of the acoustic
(sound)
wave which is generally
used to assess sound exposure to humans is the
sound pressure level
expressed in μPa or Pa. Human ear’ audible sound pressure levels
range from 20 μPa
(hearing threshold) till
20 Pa (pain threshold), resulting in the scale 1:10,000,000.
Since using such a large scale is not practical, a logarithmic
scale in decibels
(dB) was introduced which
is also in agreement with physiological and psychological
hearing sensations.

dB of
sound pressure level
(dB SPL) is defined as: 20
log10 p1/p0 where p1 is actually measured sound
pressure level of a given sound, and p0 is a reference value of
20μPa, which corresponds to the lowest
hearing threshold of the
young, healthy ear. In the
logarithmic scale the range of human ear’s audible sounds is
from 0 dB SPL (hearing threshold) to 120-140 dB SPL (pain
threshold) (see table 1 below).

The human ear is not
equally sensitive to sounds (tones) of the same sound pressure
levels but different
frequencies. This
subjective or perceived magnitude of a
sound by an individual is
called its loudness. The loudness of a sound is not equal with
its sound pressure level
and differs for different frequencies. In order to assess
loudness of a sound the isophonic curves are explored. Isophonic
curves relate the characteristic of a given tone expressed in
dB SPL to its subjective
loudness level expressed in phones (see figure 1 below). As it
could be seen in the figure below, the frequencies 3-4 kHz are
the most sensitive within
sound frequency range from
20 Hz to 20 kHz that can be heard by human ear. For frequencies
lower than 3-4 kHz and higher sound frequencies, the ear becomes
less sensitive.

While sound pressure
measurements should give a reading of the sound pressure in
dB SPL, in the context of
human hearing it is more practical to provide also a value which
corresponds more closely to the hearing sensation or loudness in
phones. The A, B, and C filters used currently in sound-level
meters were aimed at mimicking isoloudness curves over
frequency under different
conditions of sound intensities, i.e. for sounds of low, medium,
and high loudness levels, respectively (IEC 651, 1979). The “A”
network modifies the frequency response to follow approximately
the equal loudness curve of 40 phons, while the “C” network
approximately follows the equal loudness curve of 100 phons. A
“B” network is also mentioned in some texts but it is no longer
used in noise evaluations.
The popularity of the A network has grown in the course of time.
In current practice, the A- weighting curve filter is used to
weight sound pressure levels as a function of frequency,
approximately in accordance with the frequency response
characteristics of the human auditory system for pure tones.
This means that energy at low and high
frequencies is
de-emphasized in relation to energy in the mid-frequency range.

Correlation between
noise effect
hearing loss and
sound exposure levels
measured in A, B, or C weightings would not be very different. B
(or even C) weightings provide a better correspondence between
loudness and moderate (or high) acoustic levels, however A
weighting differs only from B and C as underweighting
frequencies below about
500 Hz. Since the human
ear is much more resistant
to noise-induced hearing loss (NIHL) at and by low frequencies A
weighting is more in correspondence with NIHL risk.

It should be noted that the A-filter has been adopted so
generally that sound pressure levels frequently quoted in
audiology literature simply in
dB are in fact A-weighted
levels. Many older general purpose
sound level meters are
restricted solely to A-weighted
sound pressure level
measurements.

Different decibel
measures are used in audiometry (evaluation of hearing
sensitivity) than in sound
pressure measurement. They depend on the reference value.

Pure-tone audiometric thresholds are expressed in
dB HL (hearing level) and
are referred to hearing thresholds of normal hearing young
individuals. The differences between dB HL and
dB SPL arise from
isophonic curves. Their corresponding values are given in the
table below.

2.2 What are the methods for measuring sound

Sounds are usually identified by their
frequency spectrum, which
is also relevant to human perception because the
ear analyses sounds in the
cochlea by a spectral
analysis.

The elemental component of a
frequency spectrum is a
sine wave or sinusoid with
a specific frequency. All sound waves can be described as a
linear superposition of sinusoids. Each sinusoid can be
characterized by its frequency, its amplitude and the phase in
relation to the zero-time mark. Sinusoids with the same
frequency and amplitude superimpose either constructively by
adding up to a sinusoid with double amplitude if the phase
difference is zero and destructively by cancelling out if the
phase difference is 180 degrees (or antiphase) resulting in no
sound of that
characteristic frequency at a given point.

Sound originating from
speech and music can similarly be described by their spectrum.
In general terms signals can be divided in signals with a tonal
character and with a noisy character.

Signals with a tonal character exhibit a spectrum made
up of a basic
frequency component
(f0) with harmonics (components that have a frequency which
is an integer multiple of the basis frequency (n*f0) and a
related phase.

Signals with a noisy character exhibit a spectrum
which is more complex than a linear superposition of basic
frequencies and their
harmonics.

Sound measurements are
done by determining the amplitude of the spectral components or
by detecting the sound pressure through a physical device, e.g.
a microphone. The total
sound level of a signal is
a root-sums-of-squares of the amplitude of all the spectral
components.

Signal levels, including noisy signals and music, are measured
by placing a calibrated
sound meter (SPL meter) at
the centre head location of a potential listener. This method is
generally used to determine the risk for
hearing loss in working
conditions.

The method distinguishes between various possible
measures:

The averaged level, which is the average level of all
frequency components
over a certain time period

The level measurement can be recorded by
filtering according to the A, B or C filter;
dB
(A)

The peak level indicating the highest level recorded
either of the total (weighted) signal or of specific
components

The 8-hour equivalent level
(Lequ, 8h) which is a measure for the
risk on hearing damage based on certain criteria

The method can also be used to determine the level of music in
the open field. Due to the dependence of sound waves on the
exact listening situation, as detailed in 3.2, it is clear that
this type of measurement is not suitable to head phone use where
only a small space between the head phones and the
inner ear is exposed to
sound waves.

Sound levels of signals presented through headphones are
usually measured by artificial ears. Most common are two types,
the occluded ear simulator
(OES) and the 2 cc coupler. In audiometry and hearing aid
specifications all measurements are measured using one of these
two couplers. The design of the couplers is based on the
resonance properties of the
ear canal and the
impedance of the tympanic membrane.

In the link of sound
transfer from the open field to the
ear, there is another
transfer characteristic to be included and that is the baffle
effect of head and torso. The head effects are usually
determined by using a manikin or as they are also called HATS,
head and torso simulator. It consists of a torso and head in
which artificial ears are included. The sound pressure is
measure at the eardrum. If
compared with the free field, this gives the head-related
transfer function (HRTF).

It is obvious that HATS and the couplers are based on
measurements, averaged over many torsos and ears of both genders
taking a multitude of anatomic features into account. Sound
levels in individual ears will always differ somewhat from these
values. These have to do with the following features:

Distortion of the
sound field caused by
other listeners or objects in the room

For the purpose of estimating the risk of the use of
individual music players we assume that the calculated sound
levels based on the use of artificial heads and ears are good
estimates of the real levels.

The risk for hearing damage depends on
sound or
noise level and exposure
time. Criteria were originally developed using working
conditions as a reference which are typically measured in the
open field. If we want to assess the risk of PMPs we have to
compare the levels produced by earplugs or headphones with the
measurements done in free field. This implies we have to
determine the HRTFs for the different PMPs.

The output level of a PMP is determined by using an artificial
ear. It measures the
actual sound pressure at
the eardrum. To calculate
the risk for hearing damage, the free field level has to be
calculated by using the inverse HRTF.